writeback.bt(8) System Manager's Manual writeback.bt(8)
NAME
writeback.bt - Trace file system writeback events with details. Uses
bpftrace/eBPF.
SYNOPSIS
writeback.bt
DESCRIPTION
This traces when file system dirtied pages are flushed to disk by ker-
nel writeback, and prints details including when the event occurred,
and the duration of the event. This can be useful for correlating these
times with other performance problems, and if there is a match, it
would be a clue that the problem may be caused by writeback. How
quickly the kernel does writeback can be tuned: see the kernel docs,
eg, vm.dirty_writeback_centisecs.
This uses the tracepoint:writeback:writeback_start and trace-
point:writeback:writeback_written tracepoints.
Since this uses BPF, only the root user can use this tool.
REQUIREMENTS
CONFIG_BPF and bpftrace.
EXAMPLES
Trace all writeback events with timestamps and latency details:
# writeback.bt
FIELDS
TIME Time that the writeback event completed, in %H:%M:%S format.
DEVICE Device name in major:minor number format.
PAGES Pages written during writeback.
REASON Reason for the writeback event. This may be "background", "vm-
scan", "sync", "periodic", etc.
ms Duration of the writeback event in milliseconds.
OVERHEAD
Since writeback events are expected to be infrequent (<10/sec), the
overhead of this tool is expected to be negligible (near 0%).
SOURCE
This is from bpftrace.
https://github.com/iovisor/bpftrace
Also look in the bpftrace distribution for a companion _examples.txt
file containing example usage, output, and commentary for this tool.
OS
Linux
STABILITY
Unstable - in development.
AUTHOR
Brendan Gregg
SEE ALSO
biosnoop.bt(8)
USER COMMANDS 2018-09-14 writeback.bt(8)
Generated by dwww version 1.16 on Tue Dec 16 17:18:34 CET 2025.